A composite article is an article made of one or more composite materials, e.g. materials comprising two or more components. One component is a fibre (continuous or chopped), while another component (also called a matrix) is a polymeric resin that binds the fibres together.
Composite article serve in many fields and may be found as building materials in armour plates, in aircrafts, and many others.
Before a composite article is put in service, it is important to evaluate its damage tolerance, that is, in what way defects of predetermined sizes might affect the article's stress limit, life time, etc. For this end, model articles with artificial defects are made, and their ability to stand stresses is tested.
Composite articles with artificial defects are also necessary in nondestructive testing, which is a technology regularly used for detecting defects that may be detrimental to the functioning of such articles in service. Defects appear in composite articles during manufacture, if the manufacturing protocol is not strictly followed, or during service of the article, due to stresses and environmental conditions, under which it serves. The main kind of defects are separation of layers (also called delaminations or debonding) and small voids dissipated through the entire volume in the article or a portion thereof, which is much larger than the single void. Such defect is also called porosity.
All real defects are air filled.
There are well-established requirements to detect defects of determined size (specific size depend on the specific application of the composite), and in order to do so, calibration standards with defects of predetermined size are prepared for calibrating the detection equipment.
In the state of the art, a defective composite article is prepared with Teflon disks between the layers, and these disks serve to simulate the defects. Another kind of artificial defects known in the art is flat bottom holes drilled from the back side of the article to the depth in which the defect should present.
In recent years, new technologies, such as shearography and thermography are used for non-destructive detection of defects in composite articles, and these technologies also raise the need for novel calibration standards.
Some suggestions for composite articles with artificial defects other than Teflon disks or flat bottom holes are suggested in the following publications:
JP 10-22773 describes an artificial defect detection material for non-destructive inspection. The material has glass micro balls evenly distributed in a coupling base material. The glass micro balls include gas inside them.
JP 61265565 describes a method for forming standard flaw for non-destructive inspection. According to the described method a CFRP (carbon fiber reinforced plastic) plate is cut to two portions. Holes are accurately opened to one portion thereof by machining, and columns having the same diameters are cut from the other portion. The columns are finished by a grinding means so as to have diameters slightly smaller than the dimensions of the holes. Next, force is applied to the arbitrary place of each column to form layer delaminating to said column. This column having layer delaminating is sealed so as to prevent a resin such as an epoxy resin from penetrating into the layer delaminating parts from the circumference thereof. Subsequently, an adhesive equal to that used in the plate is applied to the periphery of the column after sealing and a coated column is inserted in each hole of the plate. In this case, a fiber direction is made same to attain to prevent abnormality from the aspect of flaw detection. Thereafter, the plate and the columns are finished so as to make both surfaces thereof flat to obtain a standard test piece.